Effect of HfO~2 addition on the densification behaviours and microstructures of sintered products in the Si_3N_4-Y_2O_3-AlN system was investigated. In the HfO_2 doped specimens, the crystallization of the grain boundary phases was promoted. The densification was enhanced by the HfO_2 addition, especially in the range of 1 to 5 wt% below 1800°C. The densities of the specimens fired at 1850 and 1900°C increased with HfO_2 content. It was confirmed that the crystallisation promotion of the grain boundary phases occurred in the high temperature firing of the specimens with HfO_2 addition, which was not observed in without HfO_2. This phenomenon suggested that the full densification and the crystallisation system were carried out through the transient liquid phase sintering.
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